1. Field of the Invention
The present invention relates to a photoconductor to which high durability and a high image quality are imparted by disposing in the photoconductor, a photosensitive layer having advantageous film surface properties, high wear resistance and advantageous electrical properties; and a manufacturing method thereof. The present invention relates also to an image forming process, image forming apparatus and process cartridge for the image forming apparatus using the above-noted photoconductor having long life and high performance.
2. Description of the Related Art
Recently, organic photoconductors (OPC) are widely employed in copiers, facsimiles, laser printers, and composite apparatuses thereof owing to excellent performance and various advantages, in place of conventional inorganic photoconductors. Specific grounds thereof are thought as follows: (i) optical properties such as absorbable wavelength and absorption rate, (ii) electrical properties such as higher sensitivity and stable charging ability, (iii) margins of materials, (iv) productivity, (v) lower cost, (vi) safety, and the like.
On the other hand, photoconductors have been small-sized along with image forming apparatuses being small-sized; in addition, higher processing rate as well as maintenance free are have been required for image forming apparatuses; consequently, photoconductors are demanded for higher durability still more nowadays.
However, organic photoconductors are typically less durable since the hardness of the surface layers is relatively low due to their inherent components of charge transport substances of lower molecular mass and inactive polymers; therefore, the surface layers tend to wear significantly due to mechanical stress caused by developing systems and cleaning systems etc. under repeated usages in electrophotographic processes.
Further, rubber hardness of cleaning blades has been raised and pressure onto photoconductors applied from the cleaning blades has been increased so as to improve cleaning ability in order to enhance image quality by using toner particles with smaller particle sizes, which inevitably leading to higher wear rate of photoconductors. The wear of photoconductors certainly degrades sensitivity, electrical properties such as charging ability etc., which resulting in deteriorated images such as lower image density and background smear. Further, flaws due to local wear often bring about streak on images due to insufficient cleaning. Such wear and flaws typically dominate photoconductors in terms of lifetime to be exchanged, currently.
As such, the wear rate should be decreased in order to enhance durability of organic photoconductors, which is one of the most important objects in the art.
Previously, various proposals have been provided in order to enhance wear resistance of photosensitive layers, for example, (1) incorporation of curable binders into the photosensitive layer (e.g. Japanese Patent Application Laid-Open (JP-A) No. 56-48637), (2) employment of polymers for charge transport substances (e.g. JP-A No. 64-1728), (3) dispersing inorganic fillers into surface layers (e.g. JP-A No. 4-281461), and the like.
However, in the (1) incorporation of curable binders described above, residual voltage tends to increase owing to impurities such as polymerization initiators and/or unreacted residual groups due to insufficient compatibility with charge transport substances, thus image density tents to decrease. In the method (2) using a charge transport polymer and the method (3) using an inorganic filler, while the wear resistance of the photoconductor can be improved to some extent, a photoconductor which can fully satisfy the durability required for the organic photoconductor is not yet obtained. Further, in the method (3) using an inorganic filler, the organic photoconductor comprising an inorganic filler has such a tendency that due to a charge trap which is present on the surface of the inorganic filler, the residual electric potential of the surface layer is elevated, so that the image density is easily lowered. As such, based on these proposals (1), (2), and (3), the durability of organic photoconductors is not satisfactory on the whole, including electrical durability and mechanical durability.
Further, photoconductors containing cured product of a multi-functional acrylate monomer are proposed in order to improve the abrasion resistance and scratch resistance such as of (i) (e.g. Japanese Patent No. 3262488). In the patent literature, it is disclosed that cured material of the multi-functional acrylate monomer is included into a protective layer on photosensitive layers. However, there exist no more than simple descriptions that a charge transport substance may be contained in the protective layer and there exist no specific examples. Further, when a charge transport substance having a low molecular mass is simply added to the surface layer, it may cause problems related with the compatibility to the cured body, thereby crystallization of charge transport substance having a lower molecular mass and clouding may occur, resulting in reduction in mechanical properties.
In addition, a photoconductor is produced by way of causing reaction of monomers in a condition that a polymer binder is incorporated; therefore, there will be some problems that the curing cannot sufficiently proceed, and surface nonuniformity is induced due to phase separation at curing caused by insufficient compatibility between the cured material and the binder resin, which resulting in inferior cleaning in image forming apparatuses.
Further, another proposal is disclosed for reducing abrasion wear of photosensitive layers, in which a charge transport layer is provided using a coating liquid that comprises a monomer having a carbon-carbon double bond, a charge transport substance having a carbon-carbon double bond, and a binder resin (e.g. Japanese Patent No. 3194392). The binder resin includes a binder reactive with the charge transport substance having a carbon-carbon double bond and another binder non-reactive with the charge transport substance without having the double bond. The photoconductor allegedly represents higher wear resistance as well as proper electrical properties. However, non-reactive resins as the binder resin tend to yield surface irregularity and thus inferior cleaning, since the non-reactive resins are typically non-compatible with reaction products between the monomer and the charge transport substance, thus phase separation is likely to occur. Further, the patent literature discloses monomers having two functionalities as specific examples, which cannot bring about sufficient crosslinking density and satisfactory wear resistance due to the lower functionalities. Provided that reactive resins are employed as the binder resin, the bonding density and the crosslinking density are possibly not sufficiently high due to the lower functionalities of the monomer and the binder resin, thus electrical properties and wear resistance will not be satisfactory.
Further, another proposal is disclosed, in which photosensitive layers comprise reaction products that are produced by curing hole transport compounds having two or more functional groups capable of undergoing chain polymerization in a molecule (e.g. JP-A No. 2000-66425). However, the photosensitive layer tends to cause higher internal stress and thus to yield higher surface roughness and cracks, since the bulky hole transport compound have two or more chain polymerizable functional groups.
In this situation, the present inventors have made extensive and intensive studies with a view toward solving the above-noted problems accompanying the related art. As a result, it has been found that by producing a photoconductor in which the surface layer is a crosslinked resin layer produced by curing a radical polymerizable monomer having three or more functionalities and no charge transport structure and a radical polymerizable compound having a charge transport structure, the electrical properties and wear resistance of the photoconductor are improved. However, it was also found that such a crosslinked resin layer has not a satisfactory durability during a long-term using and depending on the crosslinking condition, the surface properties of the crosslinked resin layer are largely changed and the surface unevenness of the crosslinked resin layer becomes easily large. Accordingly, the cleaning failure of the photoconductor is easily caused and when the photoconductor is used in a long-term, the cleaning blade is locally broken and the cleaning failure is caused, so that an abnormal image in the form of a stripe is caused.